1,25-Dihydroxyvitamin D3 Attenuates the Effects of Lipopolysaccharide by Causing ADAM10-Dependent Ectodomain Shedding of Toll-Like Receptor 4.

BACKGROUND/AIMS: We investigated how 1,25-dihydroxyvitamin D3 (1,25D3) inhibits the effects of lipopolysaccharide (LPS) in human aortic endothelial cells.

METHODS: Cellular signaling was explored by determination of protein abundance with Western blot, measurement of cytosolic Ca2+ concentration and immunofluorescence staining for a disintegrin and metalloprotease 10 (ADAM10).

RESULTS: LPS stimulated the expression of intercellular adhesion molecule 1 (ICAM-1) through toll-like receptor 4 (TLR4) and subsequent activation of p38 mitogen-activated protein kinase (p38 MAPK). Pretreatment with 1,25D3 attenuated LPS-induced p38 MAPK activation and ICAM-1 expression by causing ectodomain shedding of TLR4. This effect of 1,25D3 depended on its ability to induce a rapid extracellular Ca2+ influx through L-type calcium channels because the ectodomain shedding was prevented by the absence of extracellular Ca2+ or the presence of verapamil. TLR4 ectodomain shedding was also induced by Bay K8644 (L-type calcium channel agonist). Both 1,25D3 and Bay K8644 caused extracellular Ca2+ influx-dependent ADAM10 translocation to the cell surface. Depletion of ADAM10 by siRNA transfection prevented 1,25D3- and Bay K8644-induced ectodomain shedding of TLR4, and abolished the inhibitory effect of 1,25D3 on LPS-induced ICAM-1 expression.

CONCLUSION: 1,25D3 causes ectodomain shedding of TLR4 and thereby decreases the responsiveness of cells to LPS. ADAM10, activated by extracellular Ca2+ influx, was implicated in the ectodomain cleavage of TLR4.